1. Field of the Invention:
The present invention relates to a variable strength spring, comprising a tubular body in which is situated a contact seat, concave and coaxial in respect to the tubular body, a ring made of elastomeric material coaxially engaged in the tubular body and having a checking surface turned towards said contact seat and a connection tang rigidly engaged in the elastic ring opposite the checking surface.
2. Background of the Related Art:
It is known that springs of said type are particularly suitable to be used in suspensions for transport vehicles, for example railway wagons which are required to support considerable variations of static load. Such springs must present, as an essential characteristic, a rigidity growing with the increase of static load to which they are asked to support.
Considering the behavior of suspensions in railway wagons, springs must present a relatively reduced rigidity when the wagon is unloaded (load acting on each spring equal to 800-900 kg., approximately) or when the transported load is of relatively low weight. This is a necessary condition to avoid, during running, undesirable disengagements between wheel and rail which might occur due to unavoidable unevenness, particularly in connection with junctions between rails. In conditions of a limited static load, a low strength of the spring is essential, also to avoid various impacts supported by wheels during running being entirely sent to the suspended mass.
Conversely, when the static load is of a considerable value (4200-4800 Kg.), springs must have a high strength, so that situation of static load variations does not produce excessive variations of spring flexure and, hence, excessive vertical movements of the vehicle center of gravity.
To satisfy these needs, springs which comprise essentially a shaped elastic ring are utilized; this shaped elastic ring is realized by elastomeric material of suitable rigidity, which is rigidly and coaxially engaged inside the tubular body.
The tubular body is shaped in order to present coaxially, at its interior, a concave contact seat connecting with a cylindric wall inside the tubular body. The contact seat is turned towards the elastic ring and is suitably spaced from the same.
The elastic ring is also rigidly engaged a connection tang, whose fastening portion protrudes from the ring on the side opposite to the contact seat.
In the assembly of the spring, the tubular body and the connection tang are respectively fixed to parts under relative movement, i.e. to the suspended mass and to the non-suspended mass of the vehicle, and vice versa. In such a situation the loads sent to the spring are opposed by the consequent elastic deformations of the ring.
Choosing suitable constructions for the various spring components, it it possible on confer to said spring a rigidity that within certain limits, grows with the static load increase.
When the conventional spring must support relatively reduced loads, the entire section of the deformable ring is substantially stressed by shearing stress. In such a situation the elastic ring is susceptible to deformations which are relatively high under the action of load variations and the spring shows a relatively low rigidity whose value remains substantially constant within certain limits of load. In these limits the elastic ring is deforming by moving in the tubular body without interferring with the contact seat in said tubular body. When the load increases beyond the above mentioned limits, the elastic ring, while deforming itself, goes progressively in contact, by its duly shaped checking surface with the above mentioned contact seat.
In such a situation, the elastic ring is also stressed to compression, with stresses increasing little by little while the contact surface inhibits further deformations. As a result there is a considerable increase of spring rigidity with increased load increase.
It is noted that known springs, although they allow rigidity variations with an increase of static load, have not shown, up to now, a fully satisfactory behavior.
As a matter of fact, as evident from the above, the rigidity of said springs is substantially subjected to increase with the increase of load, only after that same load has exceeded a predetermined value.